1. Field of the Invention
The invention is directed toward phenolic resin compositions which are useful in the manufacture of reinforced composites.
2. Description of the Prior Art
Phenolic resins can be broadly divided into two general classes: novolacs and resoles. Novolac resins are generally characterized as being formaldehyde deficient. That is to say that the ratio of formaldehyde to phenolic groups is <1. Resole resins are generally characterized as being formaldehyde rich. That is to say that the ratio of formaldehyde to phenolic groups is >1. Both novolacs and resoles may incorporate a variety of phenolic compounds, alone or in combination, including but not limited to phenol, resorcinol, bisphenols, phloroglucinol, cresols, alkyl phenols, phenyl ethers, tannins, and lignins. Similarly, other aldehydes may be substituted in whole or in part for formaldehyde, including but not limited to acetaldehyde, propionaldehyde, cyclohexanedicarboxaldehydes, benzaldehydes, furfural, and other aryl or heterocyclic aldehydes.
Novolac resins are usually cured (crosslinked, hardened) through the use of formaldehyde, formaldehyde-donating hardener compounds, or formaldehyde equivalent compounds. Hexa-methylenetetramine (hexa) and paraformaldehyde are often used commercially to cure novolac resins. In addition to a source of formaldehyde, heating and the presence of a catalyst are usually employed to accelerate the rate and extent of curing. Catalysts may include inorganic bases such as sodium or potassium hydroxide, Lewis acids such as zinc chloride or zinc acetate, or amines such as triethylamine.
The ideal phenolic resin for composite applications such as pultrusion, filament winding or vacuum assisted resin transfer would possess a number of key attributes:                it would be capable of being formulated to give a liquid resin at ambient or near ambient temperature using little or no inert solvents;        it would have a low free phenolic monomer content;        it would be able to utilize processing equipment made with standard materials of construction;        it would be curable through reaction with hardeners.        
Depending upon their molecular weight, resole resins can be either solids or liquids. They are cured by heating either alone or, more typically, in the presence of an acid catalyst. Composite applications are currently dominated by liquid resole resin formulations almost to the exclusion of novolacs. Yet, resoles do not possess the ideal solution because their acid catalysts require the use of processing equipment made from costly, acid-resistant materials. Furthermore, the acid catalysts necessary for curing severely limit their processing window (pot life) and storage stability. Another drawback of resoles is their relatively high solvent content needed to keep the formulation viscosity within acceptable processing parameters.
Novolac resins are typically solids that require heating in the presence of a hardener (formaldehyde source or equivalent) in order to cure to a crosslinked resin. These products are ideally suited for molding applications where the resins are compounded (blended) with hardeners, fillers, and reinforcements. However, there are certain applications, such as adhesives and composites, where it is more advantageous to use a liquid resin system. Liquid novolac resins are most often obtained in one of three ways:                by mixing the solid resins with aqueous base;        by dissolving the resin in an organic solvent;        by employing a large excess of phenolic monomer to act as a reactive diluent.        
While the water or organic solvent based formulations may be suitable for adhesive applications or for the preparation of certain prepregs, they are not desirable for composite applications where the trapped solvents lead to the generation of voids. Offgassing of solvents is not the issue with formulations containing excess phenolic monomers. Rather, the large excess of free monomers may result in undesirably long curing times or worker exposure issues.
The potential usefulness of phenolic-based fiber reinforced composites has long been recognized. The excellent flame, smoke and toxicity (FST) properties of phenolics in fire situations have made them the material of choice in the construction of aircraft interiors, rail coaches and other areas with high concentrations of people with limited egress routes. Unfortunately, phenolic polymers and their composite systems are also well known to be difficult to process and the final parts brittle.
Dailey, in two closely related U.S. Pat. Nos. 5,075,413 and 5,075,414, discusses the need for phenolic resin systems with improved processing capabilities. Specifically, he discloses the use of mixtures of phenolic resoles with resorcinol-based novolacs to provide a low temperature cure system while retaining the excellent FST properties of phenolics. He additionally discusses the use of liquid methylene donors to partially dissolve paraformaldehyde thereby reducing the overall solvent content of the resin formulations. This was anticipated to provide enhanced physical properties of the final composite parts through reduced void content.
Shea also has several patents relating to fire resistant phenolic resin compositions for use in composites. In U.S. Pat. No. 4,053,447, he discusses the use of mixtures of aldehydes with paraformaldehyde to cure resorcinol-based novolac resins. The aldehydes were said to give improved resistance to cracking and embrittlement without sacrificing FST properties. In U.S. Pat. No. 4,107,127, he discloses the use of paraformaldehyde with aqueous formaldehyde to cure resorcinol based novolac resins with improved embrittlement properties without sacrificing FST properties. Shea patents U.S. Pat. No. 4076873, EP0368927, WO 89/01013 and U.S. Pat. No. 5,202,189 are similar to the aforementioned Shea art in that they discuss resorcinol-based novolacs cured with mixtures of formaldehyde(s) and other aldehydes. In US patent application 2004/0036056, he specifically discusses the use of a four part, non-formaldehyde-containing hardener for phenolic resin systems useful for composites. His hardener consists of a non-formaldehyde methylene donor, a pH adjuster, a viscosity adjuster and a polymerization shortstop to retard the polymerization. He also claims a small amount of water in the hardener system is required for proper cure to be obtained.